The worldwide distribution of HTLV infection, the mechanisms of its transmission and its role in various types of T-cell malignancies and patients with acquired immune deficiency syndrome (AIDS) and pre-AIDS has been extensively studied. A highly sensitive enzyme linked immunosorbent assay (ELISA) has been developed to detect HTLV antibodies in sera from different donors. Using the techniques of ELISA and competition radioimmunoassays, it has been shown that: (1) HTLV-I infection is associated primarily with adult T-cell leukemia-lymphoma (ATLL). The ATLL patients frequently have lymphadenopathy, skin involvement and hypercalcemia. (2) Relatives of ATLL positive patients in the HTLV endemic areas were approximately four times more susceptible to possess HTLV antibodies than unrelated healthy donors. (3) Seroepidemiologic studies in Jamaica show a high prevalence of HTLV-I antibodies in patients with non-Hodgkins lymphoma, and chronic lymphocytic leukemia of the B-cell type. (4) In Venezuela, HTLV-I antibodies were detected (1-14%) in different regions. High HTLV antibody incidence was correlated with areas endemic for anthropod borne diseases. (5) A study of Surinam immigrants to the Netherlands show that 12% of these immigrants who are drug users have HTLV-I antibodies, whereas control Dutch drug users do not have any HTLV antibodies. (6) Studies on patients with AIDS and pre-AIDS show that approximately 85% of these patients have HTLV-III antibodies, whereas only around 10% of these patients were found to be positive for HTLV-I antibodies. (7) High levels of HTLV-I antibody were detected in African population in Ghana, Nigeria, Uganda, and South Africa. (8) Transmission studies involving baboons with malignant lymphoma from an experimental colony in Sukhumi have shown facile HTLV transmission to other baboons, macaques and owl monkeys as evidenced by seroconversion. (9) Low levels of HTLV-I antibodies have been detected in sera of Danish patients with Sezary syndrome and mycosis fungoides. (10) Monoclonal antibodies against a 52,000 dalton glycoprotein of HTLV-I have been developed. The specific antigen recognized is located on the surfaces of HTLV transformed cells.